For starters, I would like to recommend a .22 Magnum bolt action rifle. It is a great starter cartridge. Learning to shoot that well will also serve you building good habits, because the mechanics are all the the same for the big bore center fires. Breath control, trigger pull, sight picture and follow through are all the same process for any rifle. The AR platforms aren't the greatest for repeatable precision shooting. It is difficult to get a repeatable good "cheek weld" on an AR. Get and learn how to use a sling as well, a sling is an often overlooked piece of essential equipment. The .22 Magnum ammunition is a lot cheaper to learn on than a centerfire rifle and without the recoil so you can concentrate on the basics of learning your marksmanship. I would suggest the Ruger American or if you have the pockets a CZ. You will be off to a good start with either. I would add...do not dry fire a any rimfire firearm, it will damage the firing pin. Get some good snap caps for dry fire practise, dry firing is good for trigger control, sight picture and for calling your shot and seeing when the trigger releases on your target without expending ammunition.